Numerous types of centralizer for use in boreholes are known. Known centralizers are preferably made differently depending on whether they are intended for use in an open hole portion or in a cased hole portion of a borehole. A borehole comprises an "open hole" portion whose diameter varies, often considerably, and whose wall is formed by geological formations that may be soft to a greater or lesser extent, and a "cased hole" portion that is lined with metal casing and whose diameter varies little, A centralizer optimized for the cased hole portion is generally unsuitable for the open hole portion of a borehole. Conversely, a centralizer sliding against the formations is not optimized for the hard wall provided by casing.
It is thus usual to design different centralizers for different applications.
Certain centralizers are particularly adapted to centering a well tool in the casing of a borehole. An example of such a centralizer is described in U.S. Pat. No. 4,595,055 (Vannier). That device comprises wheels that bear against the wall of the casing, which wheels are mounted at the ends of hinged arms that are urged outwardly by a combination of curved resilient spring blades and coil springs. Such a centralizer operates satisfactorily in casing where the wall is made of metal and is hard. It is not adapted for sliding properly against a wall made up of geological formations, as would apply if it were to be used in an open hole portion.
Centralizers are also known that are adapted to operate in the open hole portion of a borehole. Such centralizers use curved resilient spring blades whose central portions bear slidably against the formations. In some centralizers, coil springs are provided that cooperate with the spring blades to provide a centering force that is substantially constant over a wide range of borehole diameters. The force of the coil springs is transmitted to the central portions of the spring blades by means of hinged arms.
A centralizer adapted to center a well tool in an open hole portion of a borehole is described in U.S. Pat. No. 3,915,229 (Nicolas). That device comprises a body and curved spring blades whose central portions bear against the wall of the borehole. The central portions of the blades are maintained at a uniform distance from the body by means of arms hinged on a collar that is slidably mounted on the body and that is subjected to thrust from a coil spring.
Another centralizer designed for centering a tool in the open hole portion of a borehole is described in U.S. Pat. No. 3,555,689 (Cubberly). That centralizer comprises a body on which two collars slide that are urged towards each other by a coil spring. Curved spring blades mounted between the two collars have central portions that bear against the wall of the borehole. The action of the spring blades and that of the coil springs combine to urge the blades against the wall of the borehole with a force that is substantially constant. Hinged arms mounted between the collars come into contact with the central portions of the blades to maintain them at a uniform lateral distance from the support.